Can you bind the main framebuffer's depth buffer as a depth buffer for your FBO? No. You must create a depth texture or a depth Render Buffer.
Does GL 3.0 allow using the main depth buffer? No.

How ever, when after occlusion queries, when I dump content of color and depth buffers, the result indicate the main and auxiliary depth buffers are some how shared :O

Main FBO:
Left image: color buffer, right image: 16 bit depth buffer as seen by a 8 bit viewer.

Auxiliary FBO:
Left image: color buffer, right image: 16 bit depth buffer as seen by a 8 bit viewer.

As seen from images above, the auxiliary FBO buffers has
a) aux FBO color buffer has either used main(?) depth buffer for depth test or the aux depth buffer is not cleared and content has copied from main FBO
b) aux FBO depth buffer has not been cleared before updating

As seen from code samples, the auxiliary depth buffer IS cleared before rendering and to make thinks even nastier, if main depth buffer is cleared too, the result is different (it shouldn't affect the result). If the state menagement is to trust, it thinks that
a) auxiliary FBO is binded before rendering
b) a depth buffer is attached to FBO
and
c) the read and write depth buffers are the same